The present invention relates to a spark plug, particularly of small-diameter type, for use in an internal combustion engine. Hereinafter, the term “front” refers to a spark discharge side with respect to the direction of the axis of a spark plug, and the term “rear” refers to a side opposite to the front side.
A spark plug of an internal combustion engine generally includes a metal shell and a ceramic insulator supporting therein a center electrode and a terminal electrode insulatively. The ceramic insulator is held in the metal shell by seating a stepped outer surface portion of the ceramic insulator against a protruded inner surface portion of the metal shell and crimping a rear end portion of the metal shell onto a shoulder portion of the ceramic shell. There are several methods for crimping the metal shell onto the ceramic insulator. In one crimping method, the metal shell is deformed by cold forging with an insulating powder material filled between the metal shell and the ceramic insulator as discussed in Japanese Laid-Open Patent Publication No. 2005-044627. In another crimping method (called “hot crimping”), the metal shell is deformed by plastic forming under heated conditions where the deformation resistance is low, without the use of an insulating powder material, as discussed in Japanese Laid-Open Patent Publication No. 2003-257583.
The size (diameter) reduction of the spark plug is being demanded to attain a higher degree of engine design flexibility for improvement in engine performance such as engine output and efficiency. For example, the diameter reduction of the spark plug leads to the formation of a smaller plug hole and permits the arrangement of a lager water jacket and intake/exhaust ports in the engine. Further, the spark plug is mounted in the plug hole by engaging a plug mounting tool e.g. a plug wrench on a tool engagement portion of the metal shell so that the diameter of the plug hole has to be controlled allowing for the outer diameter of the plug mounting tool. The diameter reduction of the tool engagement portion is thus particularly effective in increasing engine design flexibility.
It is however undesirable to decrease the thickness of the tool engagement portion in order to reduce the outer diameter of the tool engagement portion because the tool engagement portion is subjected to a large torsional strain during the mounting of the spark plug into the plug hole. In order to reduce the outer diameter of the tool engagement portion without decreasing the thickness of the tool engagement portion, a middle portion of the ceramic insulator, which corresponds in axial position to the tool engagement portion, could conceivably be reduced in diameter. In this case, there is no need to make a design change in a rear portion of the ceramic insulator and reduce the diameter of the rear insulator portion excessively, thereby enabling the use of a conventional plug cord and preventing an increase in the possibility of a break in the ceramic insulator.